Televisions and computer screens are used to view images. The transmissions of those images are sequenced by control signals that are sent with the image. If these control signals are not stable in relation to the image, the image will not be displayed correctly. Of particular importance in television and cathode ray tube (CRT) displays are the Horizontal and Vertical Retrace signals respectively known as Hsync and Vsync.
When a transmission is generated or processed through digital circuitry, the transmitted signals, e.g., image or control signals, can be processed incorrectly. The incorrect processing can move the signal positions relative to other portions of the transmitted signals. The resulting difference in relative position, may be influenced by a variety of variables such as the electronic circuit fabrication process or the temperature of the circuit, for example. Because display signals are repeated thousands of times per second, the interaction of the misprocessed signals can cause the position of the signal to move over time, which is known as jitter. This jitter creates a distortion on the resulting image, which is visible to viewers of the display.
In the context of television and CRT displays, the Hsync signal can suffer from jitter that shifts the Hsync signal forward or backward by at least one clocking cycle. If the jittered Hsync signal is used to time the transmission of video data, the resulting video image will be distorted. Thus because of its unreliability, the Hsync signal is not used to time the transmission of video data.